Systems and Methods for Increasing Wind Turbine Power Output

ABSTRACT

Systems and methods for increasing the power output of wind turbines in a wind farm are disclosed. In particular, a wind farm can include first and second doubly fed induction generator wind turbine systems. The rotational rotor speed of the first wind turbine system can be regulated at reduced wind speeds based at least in part on data indicative of rotor voltage to increase power output of a doubly fed induction generator. The rotor speed can be regulated such that the rotor voltage does not exceed a voltage threshold. The power output of the first wind turbine system can be further increased by reducing its reactive power output. The reduced reactive power output of the first wind turbine system can be compensated for by an increased reactive power output of the second wind turbine system.

FIELD OF THE INVENTION

The present disclosure relates generally to wind turbines, and moreparticularly to system and methods for increasing the power output of adoubly fed induction generator wind turbine system at reduced windspeeds.

BACKGROUND OF THE INVENTION

Wind power has received increased attention as being one of thecleanest, most environmentally friendly energy sources presentlyavailable. A typical modern wind turbine can include a tower, agenerator, a gearbox, a nacelle, and a rotor having one or more rotorblades. The rotor blades can transform wind energy into a mechanicalrotational torque that drives one or more generators via the rotor. Theone or more generators can be, for instance, coupled to the rotor via agearbox. The gearbox can step up the inherently low rotational speed ofthe rotor such that the generator can efficiently convert the mechanicalrotational energy to electrical energy, which can be fed into a utilitygrid via at least one electrical connection.

Wind turbines can use a variable speed operation such that the speed ofa turbine blade changes with changes in wind speed. However, as thespeed of the turbine fluctuates, the frequency of alternating currentflowing from the generator also fluctuates. Accordingly, variable speedturbine configurations can also include power converters that can beused to convert a frequency of generated electrical power to a frequencysubstantially similar to a utility grid frequency. Such power converterscan typically comprise an AC-DC-AC topology with a regulated DC link,and can be controlled by a converter controller.

Such wind turbines can use variable speed operations to optimize loadsand to improve turbine output. In particular, wind turbines are mostefficient when they operate at an optimum tip-speed ratio. Tip-speedratio is the ratio between the tangential speed of the tip of a turbineblade and the velocity of the wind at the wind turbine. Accordingly, awind turbine will collect more wind energy operating at the optimumtip-speed ratio than it will if operating outside the optimum tip-speedratio.

For many wind turbines, the operating space, and hence value to thecustomer, is limited by maximum voltages for one or more wind turbinecomponents inherent to wind turbine systems. For instance, a powerconverter in a wind turbine system can have a voltage constraint thatlimits the minimum and maximum speed values of the generator.

Thus, a need exists for systems and methods for increasing the poweroutput of a wind turbine system at reduced wind speeds while alsomaintaining power converter voltage levels within specified operatinglimits.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

One example aspect of the present disclosure is directed to a windturbine system. The wind turbine system includes a wind driven doublyfed induction generator having a rotor and a stator. The stator providesAC power to a stator bus. The wind turbine system further includes apower converter coupled to the rotor of the doubly fed inductiongenerator via a rotor bus. The power converter provides an output to aline bus. The power converter has an associated voltage threshold at therotor bus. The wind turbine system further includes a control systemconfigured to identify a reduced wind speed at the doubly fed inductiongenerator. At the reduced wind speed, the control system is furtherconfigured to regulate the rotational speed of the rotor of the doublyfed induction generator based at least in part on data indicative of therotor voltage to increase the power output of the doubly fed inductiongenerator. The control system regulates the rotational speed of therotor such that the rotor voltage does not exceed a voltage thresholdassociated with the power converter at the rotor bus.

Another example aspect of the present disclosure is directed to a methodfor increasing power output of a wind driven doubly fed inductiongenerator at reduced wind speeds. The method includes generatingalternating current power at a wind driven doubly fed inductiongenerator. The alternating current power is provided to a stator busfrom a stator of the wind driven doubly fed induction generator. Themethod further includes providing a rotor voltage from a power converterto a rotor of a wind driven doubly fed induction generator via a rotorbus. The method further includes detecting a reduced wind speed at thewind driven doubly fed induction generator. The method further includes,in response to detecting the reduced wind speed, reducing the rotationalspeed of the rotor from a first rotational speed to a second rotationalspeed based at least in part on data indicative of the rotor voltage toincrease the power output of the doubly fed induction generator. Thesecond rotational speed is determined such that the rotor voltage doesnot exceed a voltage threshold associated with the power converter atthe rotor bus.

Yet another example aspect of the present disclosure is directed to awind farm. The wind farm comprises a first doubly fed inductiongenerator having a rotor and a stator, a second doubly fed inductiongenerator having a rotor and a stator, and a control system. The controlsystem is configured to detect a reduced wind speed at the first doublyfed induction generator. In response to detecting the reduced wind speedat the first doubly fed induction generator, the control system isconfigured to control the first doubly fed induction generator to reducea reactive power output of the first doubly fed induction generator andto reduce the rotational speed of the rotor of the first doubly fedinduction generator to increase power output of the first doubly fedinduction generator. The control system is further configured to controlthe second doubly fed induction generator to increase a reactive poweroutput of the second doubly fed induction generator.

Variations and modifications can be made to these example aspects of thepresent disclosure.

These and other features, aspects and advantages of various embodimentswill become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the present disclosure and, together with thedescription, serve to explain the related principles.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 depicts an example doubly fed induction generator (DFIG) windturbine system according to example embodiments of the presentdisclosure;

FIG. 2 depicts an example controller according to example embodiments ofthe present disclosure;

FIG. 3 depicts a flow diagram of an example method for increasing thepower output of a DFIG wind turbine system according to exampleembodiments of the present disclosure;

FIG. 4 depicts an example wind farm according to example embodiments ofthe present disclosure;

FIG. 5 depicts a flow diagram of an example method for increasing thepower output of a wind farm according to example embodiments of thepresent disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

Example aspects of the present disclosure are directed to systems andmethods for increasing power output in a doubly fed induction generator(DFIG) wind turbine system at reduced wind speeds. The DFIG system caninclude a wind driven doubly fed induction generator having a rotor anda stator. The stator can provide AC power to a stator bus. The rotor canprovide AC power to a power converter via a rotor bus. The powerconverter can provide an output to a line bus. The stator bus and theline bus can be coupled to an electrical grid through a transformer,such as a two-winding transformer or a three-winding transformer.

According to example aspects of the present disclosure, the rotationalspeed of the rotor of the doubly fed induction generator can beregulated at reduced wind speeds to increase the output power of thedoubly fed induction generator system. For instance, the rotationalspeed of the rotor can be reduced to operate the wind turbine at or nearan optimum tip-speed ratio at reduced wind speeds.

Reducing the rotor speed at cut-in wind speed can provide an increasedslip at the doubly fed induction generator. In particular, slip is thedifference between the operating speed and the synchronous speed of adoubly fed induction generator divided by the synchronous speed. Theoperating speed is the rotational speed of the rotor and the synchronousspeed is the rotational speed of the magnetic field of the stator.Increased slip can cause increased rotor voltage of a doubly fedinduction generator. Rotor voltage equals locked rotor voltagemultiplied by slip. Accordingly, reduction of the rotational rotor speedof a doubly fed induction generator can provide an increased slip, whichresults in increased rotor voltage.

The lowest and highest speed points of a doubly fed induction generatorsystem are generally limited by a voltage capability of the powerconverter. Other factors can include an electric grid voltage andreactive power demand. Accordingly, at cut in wind speeds, therotational speed of the rotor of the doubly fed induction generator canbe regulated such that the power converter voltage at the rotor bus doesnot exceed the voltage capability of the power converter. Suchregulation can improve wind turbine efficiency by increasing the speedrange of the rotor at cut-in wind speed and causing the wind turbinesystem to collect more energy from the wind.

More particularly, the rotational speed of the rotor can be regulatedaccording to example aspects of the present disclosure based at least inpart on data indicative of the rotor voltage to increase the poweroutput of the doubly fed induction generator at reduced wind speeds. Forinstance, the rotational speed of the rotor can be regulated such thatthe rotor voltage does not exceed a voltage threshold associated withthe power converter at the rotor bus. The data indicative of the rotorvoltage can be determined, for instance, by one or more sensors. Forinstance, one or more sensors can be placed at the rotor bus to detect avoltage at the rotor of the doubly fed induction generator. Thisdetected rotor voltage can be used by a controller (e.g. a wind farmcontrol system and/or individual wind turbine controller) to regulatethe rotational speed of the rotor such that the rotor voltage does notexceed the voltage threshold.

The data indicative of rotor voltage can further be determined in othersuitable ways such as from the reactive power output of the doubly fedinduction generator. For instance, a lookup table can be used by acontroller to regulate the rotational rotor speed of the doubly fedinduction generator. The lookup table can define a correlation betweenvarious rotor speed points and various grid voltage and reactivepower/power factor conditions.

According to example aspects of the present disclosure, the rotor speedrange can be further increased by reducing a reactive power output ofthe doubly fed induction generator at reduced wind speeds. Suchreduction in reactive power output can be used to allow the doubly fedinduction generator wind turbine system to operate at or near theoptimum tip-speed ratio. For instance, if the rotor voltage is at thevoltage threshold but the wind turbine system is not operating at theoptimum tip-speed ratio, a further reduction in rotational rotor speedcan be necessary for the wind turbine system to operate at or near theoptimum tip-speed ratio. Accordingly, the reactive power output of thedoubly fed induction generator can be reduced to facilitate suchreduction in rotational rotor speed.

Rotor voltage can be dependent on electric grid voltage and reactivepower demand. Accordingly, reduced reactive power output at reduced windspeeds can allow for an increased reduction in rotational speed of therotor of the doubly fed induction generator without exceeding thevoltage capabilities of the power converter. The increased reduction inrotational speed of the rotor can be such that the power convertervoltage at the rotor bus does not exceed the voltage threshold.

In one example implementation, a wind farm can include a plurality ofwind turbines, such as a first wind turbine system and a second windturbine system, each coupled to an electrical grid. The wind farm canfurther include a control system configured to detect a reduced windspeed at a doubly fed induction generator of the first wind turbinesystem. For instance, the first wind turbine system can be located inthe middle of the wind farm where wind speeds can be sometimes reducedrelative to wind speeds at the perimeter of the wind farm. In responseto detecting the reduced wind speed, the control system can beconfigured to control the doubly fed induction generator of the firstwind turbine system to reduce the reactive power output of the firstwind turbine system. This can allow for an increased reduction inrotational speed of the rotor of a doubly fed induction generatorassociated with the first wind turbine system. The rotational speed ofthe rotor can be reduced such that the voltage of a power convertercoupled to the doubly fed induction generator via a rotor bus does notexceed a voltage threshold at the rotor bus.

The control system can be further configured to control the doubly fedinduction generator of the second wind turbine system to increase areactive power output of the doubly fed induction generator. Suchincrease in reactive power output can be determined based at least inpart on a reactive power demand from the electric grid. In particular,the increased reactive power output of the second wind turbine systemcan compensate for the reduced reactive power output of the first windturbine system, for instance, to meet a reactive power demand of theelectrical grid.

Referring now to the drawings, FIG. 1 depicts an example doubly fedinduction generator (DFIG) wind turbine system 100 according to exampleembodiments of the present disclosure. System 100 includes a pluralityof rotor blades 108 coupled to a rotating hub 110, which together definea propeller 106. The propeller 106 is coupled to an optional gear box118, which is, in turn, coupled to a generator 120. In accordance withaspects of the present disclosure, the generator 120 is a doubly fedinduction generator (DFIG) 120.

DFIG 120 is typically coupled to a stator bus 154 and a power converter162 via a rotor bus 156. Stator bus 154 provides an output multiphasepower (e.g. three-phase power) from a stator of DFIG 120 and the rotorbus 156 provides an output multiphase power (e.g. three-phase power)from a rotor of DFIG 120. DFIG 120 can further be coupled to acontroller 174 to control the operation of DFIG 120. It should be notedthat controller 174, in typical embodiments, is configured as aninterface between DFIG 120 and a control system 176. Controller 174 caninclude any number of control devices. In one implementation, controller174 can include a processing device (e.g. microprocessor,microcontroller, etc.) executing computer-readable instructions storedin a computer-readable medium. The instructions when executed by theprocessing device can cause the processing device to perform operations,including providing control commands to DFIG 120.

For example, as shown particularly in FIG. 2, controller 174 can includeany number of control devices. In one implementation, for example,controller 174 can include one or more processor(s) 190 and associatedmemory device(s) 192 configured to perform a variety ofcomputer-implemented functions and/or instructions (e.g., performing themethods, steps, calculations and the like and storing relevant data asdisclosed herein). The instructions when executed by the processor 190can cause the processor 190 to perform operations, including providingcontrol commands (e.g. pulse width modulation commands) to the switchingelements of the power converter 162 and other aspects of the powersystem 100. Additionally, controller 174 may also include acommunications module 194 to facilitate communications between thecontrol system 174 and the various components of the power system 100,such as any of the components of FIG. 1.

Further, the communications module 194 may include a sensor interface196 (e.g., one or more analog-to-digital converters) to permit signalstransmitted from one or more sensors to be converted into signals thatcan be understood and processed by the processors 190. It should beappreciated that the sensors (e.g. sensors 191, 193, 195) may becommunicatively coupled to the communications module 194 using anysuitable means. For example, as shown in FIG. 2, the sensors 191, 193,195 are coupled to the sensor interface 196 via a wired connection.However, in other embodiments, the sensors 191, 193, 195 may be coupledto the sensor interface 196 via a wireless connection, such as by usingany suitable wireless communications protocol known in the art. As such,the processor 190 may be configured to receive one or more signals fromthe sensors.

As used herein, the term “processor” refers not only to integratedcircuits referred to in the art as being included in a computer, butalso refers to a controller, a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit, and other programmable circuits. The processor 190 is alsoconfigured to compute advanced control algorithms and communicate to avariety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.).Additionally, the memory device(s) 192 may generally comprise memoryelement(s) including, but not limited to, computer readable medium(e.g., random access memory (RAM)), computer readable non-volatilemedium (e.g., a flash memory), a floppy disk, a compact disc-read onlymemory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc(DVD) and/or other suitable memory elements. Such memory device(s) 192may generally be configured to store suitable computer-readableinstructions that, when implemented by the processor(s) 190, configurecontroller 174 to perform the various functions as described herein.

Referring back to FIG. 1, DFIG 120 is coupled via rotor bus 156 to arotor side converter 166. Rotor side converter 166 is coupled to a lineside converter 168, which, in turn, is coupled to a line side bus 188.In example configurations, rotor side converter 166 and line sideconverter 168 are configured for normal operating mode in a three-phase,pulse width modulation (PWM) arrangement using insulated gate bipolartransistors (IGBT) switching elements. Rotor side converter 166 and lineside converter 168 can be coupled via a DC link 136 across which is a DClink capacitor 138.

Power converter 162 can also be coupled to controller 174 to control theoperation of rotor side converter 166 and line side converter 168. Inone implementation, further instructions stored in controller 174, whenexecuted by the processing device, can cause the processing device toperform operations, including providing control commands (e.g. pulsewidth modulation commands) to the switching elements of power converter162.

In typical configurations, various line contactors and circuit breakersincluding, for example, grid breaker 182 can be included for isolatingthe various components as necessary for normal operation of DFIG 120during connection to and disconnection from the electrical grid 184. Asystem circuit breaker 178 can couple a system bus 160 to a transformer180, which is coupled to an electrical grid 184 via grid breaker 182.Although transformer 180 depicts a two winding transformer, othersuitable transformers can be used, such as a three-winding transformer.In an embodiment using a three-winding transformer, the line bus 188 canbe coupled to one winding of the transformer, the stator bus 154 can becoupled to another winding of the transformer, and the grid 184 can becoupled to another winding of the transformer.

In operation, alternating current power generated at DFIG 120 byrotating rotor 106 is provided via a dual path to electrical grid 184.The dual paths are defined by stator bus 154 and rotor bus 156. On therotor bus side 156, sinusoidal multi-phase (e.g. three-phase)alternating current (AC) power is provided to power converter 162. Rotorside power converter 166 converts the AC power provided from rotor bus156 into direct current (DC) power and provides the DC power to DC link136. Switching elements (e.g. IGBTs) used in parallel bridge circuits ofrotor side power converter 166 can be modulated to convert the AC powerprovided from rotor bus 156 into DC power suitable for DC link 136.

Line side converter 168 converts the DC power on DC link 136 into ACoutput power suitable for electrical grid 184. In particular, switchingelements (e.g. IGBTs) used in bridge circuits of line side powerconverter 168 can be modulated to convert the DC power on DC link 136into AC power on line side bus 188. The AC power from power converter162 can be combined with the power from the stator of DFIG 120 toprovide multi-phase power (e.g. three-phase power) having a frequencymaintained substantially at the frequency of electrical grid 184 (e.g.50 Hz/60 Hz).

Various circuit breakers and switches, such as grid breaker 182, systembreaker 178, stator sync switch 158, converter breaker 186, and linecontactor 172 can be included in system 100 to connect or disconnectcorresponding buses, for example, when current flow is excessive and candamage components of wind turbine system 100 or for other operationalconsiderations. Additional protection components can also be included inwind turbine system 100.

DFIG 120 and power converter 162 can receive control signals from, forinstance, control system 176 via controller 174. The control signals canbe based, among other things, on sensed conditions or operatingcharacteristics of wind turbine system 100. Typically, the controlsignals provide for control of the operation of DFIG 120 and/or powerconverter 162. For example, feedback in the form of a voltage at rotorbus 156 of power converter 162 can be used to regulate the rotationalspeed of the rotor of DFIG 120. As another example, feedback in the formof sensed rotor speed of DFIG 120 can be used to control the conversionof the output power from rotor bus 156 to maintain a proper and balancedmulti-phase (e.g. three-phase) power supply. Other feedback from othersensors can also be used by controller 174 to control DFIG 120 and/orpower converter 162, including, for example, stator and rotor busvoltages and current feedbacks. Using the various forms of feedbackinformation, switching control signals (e.g. gate timing commands forIGBTs), stator synchronizing control signals, and circuit breakersignals can be generated.

According to aspects of the present disclosure, the rotational speed ofthe rotor of DFIG 120 can be regulated based at least in part on avoltage threshold associated with power converter 166 at rotor bus 156.In particular, the wind speed range of system 100 can be limited by avoltage capability of power converter 162. In particular, the minimumand maximum wind speed points of system 100 can be limited by thevoltage capability of power converter 162. The voltage thresholdassociated with power converter 162 can be determined based at least inpart on the voltage capability of power converter 162. Accordingly, atcut-in wind speed, controller 174 can be configured to control DFIG 120to reduce the rotational speed of the rotor of DFIG 120 such that thevoltage of power converter 162 at rotor bus 156 does not exceed thevoltage threshold associated with power converter 162.

For instance, for an 1800V IGBT converter, the voltage capability can bein the range of about 759 V. As used herein, the use of the term “about”in conjunction with a numerical value is intended to refer to withinabout 25% of the numerical value. This voltage constraint can causesystem 100 to lose energy at cut-in wind speed. Accordingly, at a powerfactor of 1, the rotational speed of the rotor of a 50 Hz DFIG can bereduced to 925 rpm at cut-in wind speed as opposed to 1080 rpm, at whichthe DFIG would normally operate.

FIG. 3 depicts a flow diagram for an example method (300) of increasingpower output of a DFIG wind turbine system according to exampleembodiments of the present disclosure. FIG. 3 depicts steps performed ina particular order for purposes of illustration and discussion. Those ofordinary skill in the art, using the disclosures provided herein, willunderstand that various steps of any of the methods disclosed herein canbe adapted, omitted, rearranged, or expanded in various ways withoutdeviating from the scope of the present disclosure.

At (302), method (300) can include generating alternating current powerat a wind driven doubly fed induction generator. The alternating currentpower can be provided to a stator bus from a stator of the wind drivendoubly fed induction generator. At (304), method (300) can includeproviding a rotor voltage from a power converter to a rotor of the winddriven doubly fed induction generator via a rotor bus.

At (306), method (300) can include monitoring wind speed at the winddriven doubly fed induction generator. The wind speed can be determined,for instance by an anemometer associated with the wind driven doubly fedinduction generator.

At (308), method (300) can include detecting a reduced wind speed at thewind driven doubly fed induction generator. The reduced wind speed canbe, for instance, the cut-in speed for the wind driven doubly fedinduction generator.

In response to detecting the reduced wind speed, at (310), method (300)can include controlling the rotational speed of the rotor of the winddriven doubly fed induction generator based at least in part on dataindicative of the rotor voltage. The data indicative of the rotorvoltage can include signals from one or more sensors at the rotor busand/or data in a lookup table correlating various rotor speed pointswith various grid voltage and reactive power/power factor conditions.

According to particular aspects of the present disclosure, therotational rotor speed can be reduced such that the rotor voltage doesnot exceed a voltage threshold associated with the power converter atthe rotor bus. For instance, a controller can send control commands tothe wind driven doubly fed induction generator to reduce the rotationalrotor speed of the wind driven doubly fed induction generator from afirst rotational speed to a second rotational speed. The secondrotational speed can be determined such that the wind turbine systemoperates, for instance, at or near an optimum tip-speed ratio toincrease the power output of the doubly fed induction generator. Thesecond rotational speed can further be determined such that the powerconverter voltage at the rotor bus does not exceed the voltagethreshold.

At (312), method (300) can include further controlling the rotationalspeed of the rotor of the wind driven doubly fed induction generatorbased at least in part on the voltage threshold associated with thepower converter. If the power converter voltage on the rotor sideexceeds the voltage threshold, (312) can include increasing therotational speed of the rotor to decrease the slip and the rotor voltageof the doubly fed induction generator. The rotational rotor speed can beincreased such that the power converter voltage at the rotor bus doesnot exceed the voltage threshold. If the voltage does not exceed thevoltage threshold, (312) can include maintaining the rotational speed ofthe rotor of the DFIG and method (300) can include returning to (306).

According to example aspects of the present disclosure, a plurality ofwind turbine systems, such as wind turbine system 100 depicted in FIG.1, can be a part of a wind farm. FIG. 4 depicts an example wind farm 200according to example embodiments of the present disclosure. Wind farm200 includes wind turbine system 202 and wind turbine system 204. Windturbine systems 202 and 204 can be DFIG systems, such as system 100 asdescribed in FIG. 1. Although only two wind turbine systems aredepicted, it will be appreciated by those skilled in the art that anysuitable number of wind turbine systems can be included in wind farm200. Wind turbine systems 202 and 204 can each be coupled to wind farmcontroller 206. Wind farm controller 206, in typical embodiments, isconfigured as an interface between wind farm 200 and a control system208. Wind farm controller 206 can include any number of control devices.In one implementation, wind farm control system can include a processingdevice (e.g. microprocessor, microcontroller, etc.) executingcomputer-readable instructions stored in a computer-readable medium. Theinstructions when executed by the processing device can cause theprocessing device to perform operations, including providing controlcommands to wind turbine systems 202 and 204.

Wind farm 200 can further be coupled to an electrical grid 210 via atransformer 212. Transformer 212 comprises a two-winding transformer,but it will be appreciated by those skilled in the art that variousother suitable transformers can be used, such as a three-windingtransformer. Wind farm 200 can output multiphase power (e.g. three-phasepower) to electrical grid 210 via transformer 212. The output powerlevel of wind farm 200 can be controlled at least in part by wind farmcontroller 206.

In particular, wind farm controller 206 can receive command values from,for instance, control system 208, indicative of target power values forwind farm 200. Target power values can include active power valuesand/or reactive power values. The target power values can be determinedbased at least in part on a power demand from electrical grid 210.Further, wind farm controller 206 can receive measurement dataindicative of wind farm 200 active power, reactive power, voltage,frequency at the grid connecting point, etc.

Further still, wind farm controller 206 can receive turbine measurementdata from wind turbine systems 202 and 204. For instance, turbinemeasurement data can include the frequency of power output from a DFIG,voltages, currents, active power outputs, reactive power outputs, windspeed, power factor, etc. Based at least in part on the various receiveddata, wind farm controller 206 can determine command values for windturbine systems 202 and 204. Wind turbine systems 202 and 204 can thenuse control systems included in wind turbine systems 202 and 204, suchas control system 176 depicted in FIG. 1, to control the respective windturbine systems 202 and 204 in accordance with the command values fromwind farm controller 206.

In example embodiments, wind farm controller 206 can determine commandcontrols for wind turbine systems 202 and 204 based at least in part ona lookup table. The lookup table can define a correlation betweenvarious rotor speed points and various electric grid voltage andreactive power demand conditions.

FIG. 5 depicts a flow diagram of an example method (400) for increasingthe power output of a wind farm at reduced wind speeds according toexample embodiments of the present disclosure. The rotor speed range ofa doubly fed induction generator can be further increased by reducing areactive power output at reduced wind speeds. Accordingly, at (402),method 400 can include identifying a reactive power demand of anelectrical grid. The electrical grid can be coupled to a wind farm, suchas wind farm 200 depicted in FIG. 4. The wind farm can include, forinstance, a first doubly fed induction generator and a second doubly fedinduction generator. The reactive power demand of the electrical gridcan be determined based at least in part on various grid conditionsassociated with the electrical loads coupled to the grid.

At (404), method (400) can include detecting a reduced wind speed at thefirst doubly fed induction generator. The reduced wind speed can be, forinstance, the cut-in wind speed of the doubly fed induction generator.In response to detecting the reduced wind speed at the first doubly fedinduction generator, at (406), method (400) can include reducing thereactive power output and the rotational rotor speed of the doubly fedinduction generator to increase power output of the doubly fed inductiongenerator. The rotational speed can be reduced such that the voltage ofa power converter coupled to the rotor of the doubly fed inductiongenerator does not exceed a voltage threshold at the rotor bus.

The reduced reactive power output can allow for an increased reductionin rotational rotor speed of the doubly fed induction generator. Theincreased reduction of rotational speed of the rotor can be such thatthe voltage at the rotor bus of the power converter does not exceed thevoltage threshold associated with the power converter.

At (408), method 400 can include increasing a reactive power output ofthe second doubly fed induction generator. The reactive power output ofthe second doubly fed induction generator can be determined based atleast in part on the identified electrical grid reactive power demand.For instance, the reactive power output of the second doubly fedinduction generator can be increased to compensate for the decreasedreactive power output of the first doubly fed induction generator. Theincreased reactive power output of the second doubly fed inductiongenerator can be determined such that the overall reactive power outputof the wind farm meets the required reactive power demand for the windfarm from the electrical grid.

Example

Table 1 displays example simulation results according to exampleembodiments of the present disclosure. In particular, Table 1 displayspower outputs of a doubly fed induction generator at reduced wind speedsfor different rotational rotor speeds. For instance, as indicated byTable 1, at a wind speed of 4 m/s, reducing the rotational rotor speedfrom 1065 rpm to 925 rpm provides a 31 kW increase in power output ofthe doubly fed induction generator.

TABLE 1 1065 rpm 925 rpm Difference Wind Speed [m/s] Power [kW] Power[kW] Power [kW] 3.00 5 29 24 3.50 63 90 17 4.00 129 160 31 4.50 225 23914 5.00 301 328 27 5.50 470 474 4 6.00 589 597 8

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1-14. (canceled)
 15. A wind farm, the wind farm comprising: A first windturbine system having a first doubly fed induction generator, the firstdoubly fed induction generator having a rotor and a stator; A secondwind turbine system having a second doubly fed induction generator, thesecond doubly fed induction generator having a rotor and a stator; and acontrol system, the control system configured to detect a reduced windspeed at the first doubly fed induction generator; wherein in responseto detecting the reduced wind speed at the first doubly fed inductiongenerator, the control system is configured to control the first doublyfed induction generator to reduce a reactive power output of the firstdoubly fed induction generator and to reduce the rotational speed of therotor of the first doubly fed induction generator to increase poweroutput of the first doubly fed induction generator, wherein the controlsystem further configured to control the second doubly fed inductiongenerator to increase a reactive power output of the second doubly fedinduction generator.
 16. The wind farm of claim 15, wherein the rotor ofthe first doubly fed induction generator is coupled to a power convertervia a rotor bus, the power converter having an associated voltagethreshold at the rotor bus.
 17. The wind farm of claim 16, wherein therotational speed of the rotor of the first doubly fed inductiongenerator is reduced such that the power converter voltage at the rotorbus does not exceed the voltage threshold.
 18. The wind farm of claim15, wherein the increased reactive power output of the second doubly fedinduction generator is determined based at least in part on a reactivepower demand from the electrical grid.
 19. The wind farm of claim 15,wherein the reduction in the rotational speed of the rotor of the firstdoubly fed induction generator provides increased slip at the doubly fedinduction generator.
 20. The wind farm of claim 15, wherein the reducedreactive power output of the first doubly fed induction generatorfacilitates an increased reduction in rotational speed of the rotor ofthe doubly fed induction generator such that the power converter voltageat the rotor bus does not exceed the voltage threshold.